Researchers from Project 8 are using a pioneering technique, cyclotron radiation emission spectroscopy, in their ambitious quest to determine the mass of the elusive neutrino, which holds potential implications for our understanding of the universe.

Project 8 represents a major milestone in its quest to measure neutrino mass.

The humble neutrino, an elusive subatomic particle that easily passes through ordinary matter, plays a large role among the particles that make up our universe. To fully explain how the universe came to be, we need to know its mass. But, like many of us, he avoids being weighed.

Now, an international team of researchers from the United States and Germany leading an ambitious endeavor called Project 8 reports that their remarkable strategy is a realistic contender to be the first to measure neutrino mass. Once Project 8 is fully scaled, it could help reveal how neutrinos influenced the early evolution of the universe as we know it.

In 2022, Catherine’s research team sets an upper limit on how heavy a neutrino can be. This was a monumental achievement that took decades to complete. But these results simply narrow the search window. Katherine will soon reach its target detection limits, and may one day exceed them, but the featherweight neutrino may be even lighter, which begs the question: “What’s next?”

Cyclotron radiation emission spectroscopy

Cyclotron radiation emission spectroscopy (CRES), shown here, is the key to an entirely new method aimed at determining the mass of the elusive neutrino. Credit: Alec Lindemann, Project 8 Team

Ghostly particle tracking

In their latest study, the project team 8 reports in the journal Physical review letters That they can use a completely new technology to reliably track and record a natural event called beta decay. Each event releases a tiny amount of energy when a rare radioactive form of hydrogen — called tritium — decays into three subatomic particles: a helium ion, an electron, and a neutrino.

The ultimate success of Project 8 depends on an ambitious plan. Instead of trying to detect a neutrino – which easily passes through most detector technologies – the research team instead followed a simple measurement strategy that can be summarized as follows:

We know the total mass of tritium corn The energy of its parts is equal thanks to Einstein. When we measure a free electron from beta decay, and know its total mass, the “missing” energy is the neutrino’s mass and motion.

“In principle, as the technology develops and scales, we have a realistic chance of reaching the scale needed to determine neutrino mass,” said Brent Vandevender, one of the principal investigators on Project 8 at the Department of Energy’s National Pacific Northwest Center. Lab.

Why Project 8?

These researchers chose to pursue an ambitious strategy because they weighed the pros and cons and concluded that it could work.

Talia Weiss is a graduate student in nuclear physics at Yale University. She and her project colleagues have spent 8 years figuring out how to accurately extract electron signals from electronic background noise. Christine Claessens is a postdoctoral fellow at University of Washington Who obtained a Ph.D. In Project 8 at the University of Mainz, Germany. Weiss and Claessens performed the final analyzes that set limits on the neutrino mass derived from the new technique for the first time.

“The neutrino is incredibly light,” Weiss said. “It’s more than 500,000 times lighter than an electron. So, when neutrinos and electrons are created at the same time, the mass of the neutrino has only a tiny effect on the electron’s motion. We want to see that small effect. So, we need an ultra-precise way to measure how fast Electrons move.”

Project 8 relies on such a technique, one pioneered more than a decade ago by physicists Joe Formaggio and Ben Monreal, then working at MIT. An international team came together around the idea and formed Project 8 to transform the vision into a practical tool. The resulting method is called cyclotron radiation emission spectroscopy (CRES). It captures microwave radiation emitted by newborn electrons as they spin in a magnetic field. These electrons carry most, but not all, of the energy released during a beta decay event. It’s that missing energy that can reveal the neutrino’s mass. This is the first time that tritium beta decay, setting an upper limit on neutrino mass, has been measured using the CRES technique.

How can scientists weigh neutrinos? Credit: Animation by Sarah Levine for Pacific Northwest National Laboratory

Innovative approaches and challenges

The team is only interested in tracking these electrons because their energy is key to detecting the neutrino’s mass. While this strategy has been used previously, the CRES detector measures crucial electron energy with the potential to scale beyond any existing technology. And this scalability is what sets Project 8 apart. Elise Nowitzki is an assistant professor at the University of Washington and led many aspects of the newly published work.

“Nobody does this,” Nowitzki said. “We’re not taking an existing technology and trying to tweak it a little bit. We’re kind of in the Wild West.”

In their latest experiment, conducted at the University of Washington in Seattle, the team tracked 3,770 tritium beta decay events over an 82-day experimental period in a sample cell the size of a single pea. The sample cell is cryogenically cooled and placed in a magnetic field that traps the emerging electrons long enough for the system’s recording antennas to record the microwave signal.

Most importantly, the team did not record any false signals or background events that could be mistaken for the real thing. This is important because even a very small background can obscure the neutrino mass signal, making the useful signal more difficult to interpret.

From chirps to signals

A subset of Project 8 researchers, led by experimental physicist Noah Oblath of PNNL, but with dozens of others from multiple institutions, has developed a set of specialized programs — each named after different insects.(1)– Taking raw data and converting it into signals that can be analyzed. The project engineers have put on their tinkering hats to invent the different parts that make Project 8 come together.

“We have engineers who play a critical role in this effort,” Novitzky said. “It’s kind of weird from an engineer’s point of view. Experimental physics is kind of the border between physics and engineering. You have to get enterprising engineers and practical-minded physicists to collaborate and make these things come into being because these things aren’t in the textbooks.”

Reach the finish line

Now that the team has demonstrated its design and experimental system that works using tritium particles, it has another pressing task ahead of it. A subset of the full team is now working on the next step: a system that produces, cools and traps individual tritium atoms. This step is difficult because tritium, like its more abundant cousin, hydrogen, prefers to form molecules. These molecules would make Project Team 8’s ultimate goals unattainable. The researchers, led by physicists at the University of Mainz, are developing a test bed to create and trap atomic tritium using complex arrays of magnets that will prevent it from even touching the sample cell walls – where it will almost certainly return to the molecular level. Form.

This technological advance, and scaling up the entire device, will be the critical steps to reaching and eventually surpassing the sensitivity achieved by Catherine’s team.

Currently, the research team, which includes contributing members from ten research institutions, is testing designs to scale the experiment from a pea-sized sample chamber to a chamber 1,000 times larger. The idea is to capture more beta decay events with a larger listening device, from the size of a pea to the size of a beach ball.

“Project 8 is not only a bigger and better CRES experiment, it is the first CRES experiment and the first experiment ever to use this detection technology,” Oblath said. “I’ve never done that before. Most experiments have a history of 50 or 100 years, at least for the detection technology they use, whereas this is quite new.


  1. The suite of software specially developed by investigators working on Project 8 includes Morpho, Locust, Katydid, Psyllid, and Dragonfly.

Reference: “Tritium beta spectrometry and neutrino mass limit from cyclotron radiation emission spectroscopy” by A. Ashtari-Isfahani et al. (Project 8 Cooperation), September 6, 2023, Physical review letters.
doi: 10.1103/PhysRevLett.131.102502

Each project team investigator brings 8 complementary skills to the team effort. The full list of collaborators can be found on the Project 8 website.

Project 8 is supported by the US Department of Energy’s Office of Science, the Office of Nuclear Physics, the National Science Foundation, the German research foundation PRISMA+ Cluster of Excellence, and internal investments by all collaborating institutions.

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